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

oenius/OHHTTPStubs,SummerHanada/OHHTTPStubs,ThomWee/OHHTTPStubs,oenius/OHHTTPStubs,kazmasaurus/OHHTTPStubs,JinlianWang/OHHTTPStubs,sessuru/OHHTTPStubs,virajs/OHHTTPStubs,hirohisa/OHHTTPStubs,liufeigit/OHHTTPStubs,SummerHanada/OHHTTPStubs,hirohisa/OHHTTPStubs,JinlianWang/OHHTTPStubs,virajs/OHHTTPStubs,tarbrain/OHHTTPStubs,SummerHanada/OHHTTPStubs,ThomWee/OHHTTPStubs,tarbrain/OHHTTPStubs,oenius/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,manuyavuz/OHHTTPStubs,hirohisa/OHHTTPStubs,tarbrain/OHHTTPStubs,vladzz/OHHTTPStubs,kazmasaurus/OHHTTPStubs,liufeigit/OHHTTPStubs,manuyavuz/OHHTTPStubs,sessuru/OHHTTPStubs,virajs/OHHTTPStubs,bruceflowers/OHHTTPStubs,kazmasaurus/OHHTTPStubs,bruceflowers/OHHTTPStubs,bruceflowers/OHHTTPStubs,JinlianWang/OHHTTPStubs,sessuru/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,hirohisa/OHHTTPStubs,ThomWee/OHHTTPStubs,manuyavuz/OHHTTPStubs,oenius/OHHTTPStubs,ThomWee/OHHTTPStubs,SummerHanada/OHHTTPStubs,tarbrain/OHHTTPStubs,liufeigit/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,MaxGabriel/OHHTTPStubs,Labradorom/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,virajs/OHHTTPStubs,JinlianWang/OHHTTPStubs