Headers API constructor and methods

- Reworked the append method to support the inner `hyper::header::Headers`'s HashMap `insert` method, which overwrites entries instead of appending.
- Filled out constructor as well as delete, get, has, and set methods.
- Updated relevant test expectations

tests/wpt/metadata/fetch/api/headers/headers-basic.html.ini

@@ -1,11 +1,5 @@
 [headers-basic.html]
   type: testharness
-  [Create headers from no parameter]
-    expected: FAIL
-
-  [Create headers from undefined parameter]
-    expected: FAIL
-
   [Create headers from empty object]
     expected: FAIL
 
@@ -15,7 +9,7 @@
   [Create headers with OpenEndedDictionary]
     expected: FAIL
 
-  [Create headers whith existing headers]
+  [Create headers with existing headers]
     expected: FAIL
 
   [Check append method]
@@ -47,4 +41,3 @@
 
   [Check forEach method]
     expected: FAIL
-

tests/wpt/metadata/fetch/api/headers/headers-errors.html.ini

@@ -1,44 +1,7 @@
 [headers-errors.html]
   type: testharness
-  [Check headers get with an invalid name invalidĀ]
-    expected: FAIL
-
-  [Check headers get with an invalid name [object Object\]]
-    expected: FAIL
-
-  [Check headers delete with an invalid name invalidĀ]
-    expected: FAIL
-
-  [Check headers delete with an invalid name [object Object\]]
-    expected: FAIL
-
-  [Check headers has with an invalid name invalidĀ]
-    expected: FAIL
-
-  [Check headers has with an invalid name [object Object\]]
-    expected: FAIL
-
-  [Check headers set with an invalid name invalidĀ]
-    expected: FAIL
-
-  [Check headers set with an invalid name [object Object\]]
-    expected: FAIL
-
-  [Check headers set with an invalid value invalidĀ]
-    expected: FAIL
-
-  [Check headers append with an invalid name invalidĀ]
-    expected: FAIL
-
-  [Check headers append with an invalid name [object Object\]]
-    expected: FAIL
-
-  [Check headers append with an invalid value invalidĀ]
-    expected: FAIL
-
   [Headers forEach throws if argument is not callable]
     expected: FAIL
 
   [Headers forEach loop should stop if callback is throwing exception]
     expected: FAIL
-

tests/wpt/metadata/fetch/api/headers/headers-structure.html.ini

@@ -1,3 +1,11 @@
 [headers-structure.html]
   type: testharness
-  expected: TIMEOUT
+  expected: OK
+  [Headers has entries method]
+    expected: FAIL
+
+  [Headers has keys method]
+    expected: FAIL
+
+  [Headers has values method]
+    expected: FAIL