Fix race conditions in Faraday::Utils::Headers

Faraday::Utils::Headers uses a shared 'KeyMap' hash to cache
symbol -> string mappings of HTTP header names. When multiple threads
are writing to the cache, race conditions may occur, as the Hash class
is not thread-safe in Ruby.

This commit adds a mutex to synchronize writes to the KeyMap hash.
Merge pull request #250 from leocassarani/keymap-mutex

Fix race conditions in Faraday::Utils::Headers