Correct the default Content-Type for XHR Post with string data as per XHR specification.

As per the W3C XMLHttpRequest specification (http://www.w3.org/TR/XMLHttpRequest/#the-send%28%29-method)
the default data type for string data is "text/plain;charset=UTF-8" when content-type
is not set by the author. However "application/xml" was set as the default type.
This resulted in preflight requests being made for string data
when the content-type is not set explicitly using setRequestHeader() since "application/xml"
is not in mime-type whitelist (http://www.w3.org/TR/cors/#simple-header).

Updated the content-type as per XHR specification.
Added layout test to verify no preflight test is made when data is string in XHR POST.
Added layout test to verify "text/plain;charset=UTF-8" is set as content-type
for XHR POST with string data.
Updated the test expecations of the impacted tests.

BUG=71694

R=abarth@chromium.org, cevans@chromium.org, bbudge@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22601002

git-svn-id: svn://svn.chromium.org/blink/trunk@155717 bbb929c8-8fbe-4397-9dbb-9b2b20218538