Improve regular expression to detect the charset

The current regular expression does not detect most of the charsets from the content-type header. This regular expression detect the following cases:

* text/html; charset="iso-8859-1"
* text/html; charset=iso-8859-1
* text/html; charset=ISO-8859-15
* text/plain;  	charset="iso-8859-1"
* text/plain;  	charset="us-ascii"
* text/plain;  	charset="utf-8"
* text/plain;  	charset="windows-1255"
* text/plain;  	charset=ISO-8859-1;  	format="flowed"
* text/plain; charset=ISO-8859-1; delsp=yes; format=flowed
* text/plain; delsp=yes; charset=ISO-8859-1; format=flowed
* text/plain; format=flowed; charset="iso-8859-1";  	reply-type=original
* text/plain; format=flowed; delsp=yes; charset=iso-8859-15

The current regex only detect the three of them. This Content-Type-Headers are real world headers from the TYPO3 mailing list.
Merge pull request #1 from andygrunwald/patch-1

Improve regular expression to detect the charset