Auto merge of #9631 - jongiddy:remove-extra-mask-byte, r=jdm

Validate MIME pattern checking

The MIME detector for a HTML file contains an additional byte in the mask.  While it doesn't hurt, it is not used in the byte matching code (since it is zipped with the `pattern` iterator).

I'm not clear how to test this within the `servo` code. The fix doesn't change any externally visible behavior. A test to validate the `ByteMatcher` structures would require the private ByteMatcher structures to be visible to the test. I could use a sub-module, as described at https://doc.rust-lang.org/book/testing.html#the-tests-module but this pattern doesn't appear anywhere else in the `servo` code.

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tests/unit/net/mime_classifier.rs

@@ -50,6 +50,12 @@ fn test_sniff_mp4_matcher_long() {
     assert!(matcher.matches(&data));
 }
 
+#[test]
+fn test_validate_classifier() {
+    let classifier = MIMEClassifier::new();
+    classifier.validate().expect("Validation error")
+}
+
 #[cfg(test)]
 fn test_sniff_with_flags(filename_orig: &path::Path,
                                  type_string: &str,