This commit modifies the headers API script to correctly validate value. As a result of this change, more wpt tests pass. The commit also changes the expected test results.
- 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
