Allow possibility of not setting the response['headers']['Content-Type'] in contracts

[kga20]

Expected - when creating contracts, we should be able to not set the response['headers']['Content-Type'] and it shouldn't be defaulted to 'application/json'

Actual - Even when we don't specify .withResponseHeaders() function, the contract has
"response": {
"headers": {
"Content-Type": [
"application/json;charset=UTF-8"
]
},

Steps:
We use pact for c++ api to write our contracts ..
sample code for generating contract
m_pact.given("some provider state")
.uponReceiving("a request to xxx")
.withRequest("POST", "/abc")
.withJsonBody(requestMatcher)
.willRespondWith(200)
.withResponseJsonBody(responseMatcher);

We expect this not to set a default response['headers']['Content-Type']
and we should explicitly be setting it ..