[Here is the result of my research.]
RFC 2616 clause 3.4 says the following:
HTTP character sets are identified by case-insensitive tokens. The
complete set of tokens is defined by the IANA Character Set registry
[19].charset = token
The IANA Character Set registry is now maintained here. At the very top of this document under Note, the second paragraph reads:
The character set names may be up to 40 characters taken from the
printable characters of US-ASCII. However, no distinction is made
between use of upper and lower case letters.
Conclusion: These two references indicate that case does not matter when specifying a character set name.