RFR 8210285 : CharsetDecoder/Encoder's constructor does not reject NaN

[Ivan Gerasimov]

Hello!

The javadoc for CharsetDecoder [1] states that an exception is thrown 
when a non-positive number is passed in as an argument.

However we only reject negative or zero numbers, but not NaN.

And likewise for CharsetEncoder.

Would you please help review a trivial fix?

BUGURL: https://bugs.openjdk.java.net/browse/JDK-8210285
WEBREV: http://cr.openjdk.java.net/~igerasim/8210285/00/webrev/

Thanks in advance!

[1] 
https://download.java.net/java/early_access/jdk11/docs/api/java.base/java/nio/charset/CharsetDecoder.html#%3Cinit%3E(java.nio.charset.Charset,float,float)

-- 
With kind regards,
Ivan Gerasimov