Normalise HTTP headers

[jnioche]

Port SpellCheckedMetadata from Nutch to cater for variations returned by servers

[jnioche]

Note : should probably reuse org.apache.http.HttpHeaders instead of having our own version. Need to check whether the Apache http core library has any mechanism for normalising the keys returned by a server.

[rzo1]

Note : should probably reuse org.apache.http.HttpHeaders instead of having our own version. Need to check whether the Apache http core library has any mechanism for normalising the keys returned by a server.

This was implemented with #1588 - porting Spellcheck is still open.