Limit the maximum request post size to 10 MB (megabytes).

std::numeric_limits<qint64>::max is too big for QByteArray (throws Out of Memory exception).
Set up the limit like it was done in Google Chrome
Ref: https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/core/inspector/NetworkResourcesData.cpp

Related to issue #10158 https://github.com/ariya/phantomjs/issues/10158