Fix FrozenError when parsing ERB files in Ruby 4.0

[karinevieira]

Running rake gettext:find on Ruby 4.0 projects fails with the following error:

FrozenError: can't modify frozen String: "#coding:UTF-8\n_erbout = +''; _erbout.<< "<h2>".freeze;

In Ruby 4.0, ERB#src now returns a frozen String. GetText::ErbParser#parse calls src.force_encoding(encoding) directly on that value, which raises a FrozenError.

To fix that, we're using the unary + operator (+erb.src) to get a mutable copy of the string.

[kou]

Thanks!