Handle all multiple response headers as a table
Changed test to just check for multiple header values
Added test for case insensitive request headers
Moved header metatable to a separate module
Push request headers into http_headers object for normalisation
Normalising metatable needs to be created on the fly so that we can have multiple non-overlapping instances.
Tests for header access in all cases
Test that underscored syntax is converted to hyphens on write
Documented changes to header behaviour
Third time lucky with metatable logic. We now ensure our header fields are unique