Fix FileWriterWithEncoding.Builder ignoring any Charset but the default

[lenamonj]

FileWriterWithEncoding.Builder seeds its encoder from the default Charset, and getEncoder() throws IllegalStateException when Charset and encoder disagree. So setCharset(UTF_16) throws at get(), and setCharsetEncoder(UTF_16.newEncoder()) throws too, because the other half is still the default: the builder could not write any encoding but the platform default. The class Javadoc's Charset example also passes a Charset to setCharsetEncoder, which does not compile.

The builder now keeps the pair consistent as ReaderInputStream.Builder does: setCharset refreshes the encoder, setCharsetEncoder sets the Charset, and the mismatch guard, with nothing left to catch, is removed. The example calls setCharset.

Tests: testBuilder_nonDefaultCharset and testBuilder_nonDefaultCharsetEncoder, red on master and green with the change. Default Maven goal green.

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