Bug 1125963 - Part 1: Fix serialization of the pre-wrap elements that Thunderbird relies on; r=bzbarsky

This ensures that the plaintext serializer doesn't use the preformatted
text code path if we have encountered a pre-wrap element that
Thunderbird uses (which means setting white-space: pre-wrap and width:
NNch on the body element.)

It also ensures that we use 0 as the wrap column number passed down to
the plaintext serializer, instead of -1, which this code seems to be
unable to handle properly.

dom/html/HTMLTextAreaElement.cpp

@@ -1480,7 +1480,7 @@ HTMLTextAreaElement::GetWrapCols()
   nsITextControlElement::GetWrapPropertyEnum(this, wrapProp);
   if (wrapProp == nsITextControlElement::eHTMLTextWrap_Off) {
     // do not wrap when wrap=off
-    return -1;
+    return 0;
   }
 
   // Otherwise we just wrap at the given number of columns