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/HTMLInputElement.cpp

@@ -7219,7 +7219,7 @@ HTMLInputElement::GetCols()
 NS_IMETHODIMP_(int32_t)
 HTMLInputElement::GetWrapCols()
 {
-  return -1; // only textarea's can have wrap cols
+  return 0; // only textarea's can have wrap cols
 }
 
 NS_IMETHODIMP_(int32_t)