[Minor] Header labels could be properly cased

[MrTrick]

In the response headers, couchdb uses 'Content-Type', and couchdb-lucene uses 'content-type'.
According to the RFC 4.2, header field names are case-insensitive, but it being different from couchdb tripped up the library I'm using.

[rnewson]

This is caused by Python's httplib module, I think.

I've applied a fix and verified that headers are returned in the expected case.