1.6.0.6 Method chaining support for HttpClient

Added method chaining support for HttpClient for header setter methods. so now methods clearAllRequestProperties(), removeRequestProperty(), setContentType() and setRequestHeader() return the instance of HttpClient so they could be chained. So now it is possible to write:

new HttpClient()
.setContentType(...)
.setRequestHeader(...)
.setRequestHeader(...)
.setRequestHeader(...)
.sendHttpRequest(...);