Auto merge of #9598 - TimNN:xhr-header-step-3, r=Ms2ger

Implement XHR::SetRequestHeader Step 3

Closes #9548.

Alternative implementation of #9595.

cc @timvandermeij

I'm not sure if a utility method on `ByteString` is the best place for this functionality, maybe a free function in XHR module would be more suitable.

Also where would be the correct place to add a test for this functionality (if required)?

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components/script/dom/bindings/str.rs

@@ -48,76 +48,6 @@ impl ByteString {
     pub fn to_lower(&self) -> ByteString {
         ByteString::new(self.0.to_ascii_lowercase())
     }
-
-    /// Returns whether `self` is a `token`, as defined by
-    /// [RFC 2616](http://tools.ietf.org/html/rfc2616#page-17).
-    pub fn is_token(&self) -> bool {
-        is_token(&self.0)
-    }
-
-    /// Returns whether `self` is a `field-value`, as defined by
-    /// [RFC 2616](http://tools.ietf.org/html/rfc2616#page-32).
-    pub fn is_field_value(&self) -> bool {
-        // Classifications of characters necessary for the [CRLF] (SP|HT) rule
-        #[derive(PartialEq)]
-        enum PreviousCharacter {
-            Other,
-            CR,
-            LF,
-            SPHT, // SP or HT
-        }
-        let mut prev = PreviousCharacter::Other; // The previous character
-        self.0.iter().all(|&x| {
-            // http://tools.ietf.org/html/rfc2616#section-2.2
-            match x {
-                13  => { // CR
-                    if prev == PreviousCharacter::Other || prev == PreviousCharacter::SPHT {
-                        prev = PreviousCharacter::CR;
-                        true
-                    } else {
-                        false
-                    }
-                },
-                10 => { // LF
-                    if prev == PreviousCharacter::CR {
-                        prev = PreviousCharacter::LF;
-                        true
-                    } else {
-                        false
-                    }
-                },
-                32 => { // SP
-                    if prev == PreviousCharacter::LF || prev == PreviousCharacter::SPHT {
-                        prev = PreviousCharacter::SPHT;
-                        true
-                    } else if prev == PreviousCharacter::Other {
-                        // Counts as an Other here, since it's not preceded by a CRLF
-                        // SP is not a CTL, so it can be used anywhere
-                        // though if used immediately after a CR the CR is invalid
-                        // We don't change prev since it's already Other
-                        true
-                    } else {
-                        false
-                    }
-                },
-                9 => { // HT
-                    if prev == PreviousCharacter::LF || prev == PreviousCharacter::SPHT {
-                        prev = PreviousCharacter::SPHT;
-                        true
-                    } else {
-                        false
-                    }
-                },
-                0...31 | 127 => false, // CTLs
-                x if x > 127 => false, // non ASCII
-                _ if prev == PreviousCharacter::Other || prev == PreviousCharacter::SPHT => {
-                    prev = PreviousCharacter::Other;
-                    true
-                },
-                _ => false // Previous character was a CR/LF but not part of the [CRLF] (SP|HT) rule
-            }
-        })
-    }
 }
 
 impl Into<Vec<u8>> for ByteString {