Add JSON Content-Type header to request

[ajm13]

RCSB sequence search is currently returning a 415 status code for this library due to the lack of Content-Type header in the request. This PR adds the required header.

Can be verified with a simple sequence query:

import pypdb
query = pypdb.Query('SLDRSSCFTGSLDSIRAQSGLGCNSFRY', query_type='sequence', return_type='entry')
print(query.search())