Fix bug when passing nil headers to JSON convenience method
Fix bug when passing nil headers to JSON convenience method

tarbrain/OHHTTPStubs,tarbrain/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,danielctull-forks/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,tarbrain/OHHTTPStubs,tarbrain/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,danielctull-forks/OHHTTPStubs,danielctull-forks/OHHTTPStubs,danielctull-forks/OHHTTPStubs