dialyzer unmatched warnings

There are several unmatched warnings in the ibrowse_http_client and ibrowse_test modules. These warnings should be easy to fix and make sense to fix.

The line numbers are based on the #75 pull request commit (https://github.com/norton/ibrowse/commit/1a809408917db788c679c2a0ee3bdddf86f29e92)

ibrowse_http_client.erl:197: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_http_client.erl:298: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_http_client.erl:322: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_http_client.erl:335: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_http_client.erl:352: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_http_client.erl:576: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_http_client.erl:581: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_http_client.erl:584: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_http_client.erl:700: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_http_client.erl:783: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_http_client.erl:791: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_http_client.erl:1394: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_http_client.erl:1850: Expression produces a value of type 'ok' | integer(), but this value is unmatched
ibrowse_test.erl:83: Expression produces a value of type atom() | tid(), but this value is unmatched
ibrowse_test.erl:84: Expression produces a value of type atom() | tid(), but this value is unmatched
ibrowse_test.erl:85: Expression produces a value of type atom() | tid(), but this value is unmatched
ibrowse_test.erl:153: Expression produces a value of type 'ok' | integer(), but this value is unmatched
ibrowse_test.erl:197: Expression produces a value of type 'ok' | {'error',atom()}, but this value is unmatched
ibrowse_test.erl:243: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_test.erl:244: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_test.erl:245: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
ibrowse_test.erl:247: Expression produces a value of type 'ignore' | {'error',_} | {'ok',pid()}, but this value is unmatched
ibrowse_test.erl:392: Expression produces a value of type 'ok' | {'error','unknown_req_id'}, but this value is unmatched
ibrowse_test.erl:399: Expression produces a value of type 'ok' | {'error','unknown_req_id'}, but this value is unmatched