Day 11: Append is Still Almost There

Things we did:

• Addressed all the feedback from PR#2. Tests appear to be passing. Fingers crossed!

TODO:

• Modify xmlhttprequest.rs to use our is_forbidden_header_name() function
• Come to a conclusion regarding the field-content production, i.e. here.
• Write tests for our append method and associated helper functions
• Start working on other Headers API methods, such as delete, get, set, has.