[JAVA] fix "text/plain" with charset specification

[martin-mfg]

fix #15310
fix #12616

Reported problem occurs because the checks for "text/plain" Content Type did not account for the possibility that the Content Type additionally specifies the charset. i.e. "text/plain;charset=UTF-8" was not properly recognized as "text/plain" case.

According to the Media Type specification, we can ignore everything after the first ";", so that's what my fix does.

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