Key Headers HashMap with SendStr instead of &'static str

[reem]

This fixes an unsafe transmute that could cause use-after-frees
on collision resolution in the headers hash. The new scheme allocates
Strings on insertion, but prevents most allocations on lookup by using
the equiv methods for querying by &str.

Fixes #12

[seanmonstar]

I'm thinking keying with SendStr would be best. Keeping the header_name fn
returning static slices is ergonomic, and internally it can be wrapped into
a Slice. No need to allocate those strings for every single request. And no
user has to touch that HashMap, as it's Internal.

[reem]

@seanmonstar You are right. I've updated the PR and commit.