html/template: MIME types handling is not case-insensitive

[stjj89]

Section 5.1 of RFC 2045 states that "[the MIME] type, subtype, and parameter names are not case sensitive." However, MIME types in the type attribute of script elements are not handled in a case insensitive manner in html/template. The following template:

<script type="text/javascript">{{"Hello"}}!</script>
<script type="TEXT/JAVASCRIPT">{{"Hello"}}!</script>

produces the following output

<script type="text/javascript">"Hello"!</script>
<script type="TEXT/JAVASCRIPT">Hello!</script>

The problem is that the type value is read directly from the template here and is never converted to lowercase when checked here.

[gopherbot]

CL https://golang.org/cl/40702 mentions this issue.