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
Discussion: https://github.com/ariya/phantomjs/commit/3edcabef9f36636adcf05926351a3fb09c781e0d