Make charset parameter name comparison case-insensitive

[rootkiller6788]

Problem

HttpHeaderParser.parseCharset() only matches the exact lowercase charset parameter name when parsing a Content-Type header:

if (pair[0].equals("charset")) {
    return pair[1];
}

HTTP header parameter names are case-insensitive (RFC 7230 section 3.2.6). A server that emits e.g. Content-Type: text/plain; Charset=UTF-8 or CHARSET=UTF-8 therefore has its charset silently ignored, and the response body falls back to the default encoding. For StringRequest the default is ISO-8859-1, so any non-ASCII text (e.g. UTF-8 JSON or localized content) is decoded as mojibake.

Fix

Compare the parameter name with equalsIgnoreCase so charset, Charset, and CHARSET are all recognized, matching the case-insensitivity of the rest of header handling in Volley (header names are already looked up case-insensitively via a TreeMap with String.CASE_INSENSITIVE_ORDER).

Verification

• Reproduced the bug against the unmodified source with a standalone harness: Content-Type: text/plain; Charset=UTF-8 returned ISO-8859-1; after the fix it returns UTF-8. CHARSET=Shift_JIS is also recognized.
• Added test coverage to HttpHeaderParserTest.parseCharset() for mixed- and upper-case charset parameter names.