Under certain situations Solr produces managed schema XML that cannot be loaded

While persisting the ManagedIndexSchema as XML, non-printable characters in field names get escaped as #nn;, where nn is the decimal representation of the non-printable character. For example, if the field name has the byte 0x14, it gets escaped as #20;. This in indistinguishable from the literal #20; in the field name. If we have two fields - one with the non-printable character and the other with the literal string, two fields get generated with the same name. Loading the resulting XML, naturally, causes an exception. To fix this, any occurrence of literal # in the field name should be escaped, with say ##.
A second problem is that while escaping happens when generating XML, the corresponding unescaping does not happen on loading it. This asymmetry should be fixed as well.