Avoid modifying frozen string literals (#1364)

In particular, the extconf.rb build script. But there's also one test as well.

Author: pat

ext/mysql2/extconf.rb

@@ -86,7 +86,7 @@ def add_ssl_defines(header)
   /usr/local/opt/mysql@*
   /usr/local/opt/mysql-client
   /usr/local/opt/mysql-client@*
-].map { |dir| dir << '/bin' }
+].map { |dir| "#{dir}/bin" }
 
 # For those without HOMEBREW_ROOT in PATH
 dirs << "#{ENV['HOMEBREW_ROOT']}/bin" if ENV['HOMEBREW_ROOT']

spec/mysql2/client_spec.rb

@@ -886,7 +886,7 @@ def run_gc
     end
 
     it "should carry over the original string's encoding" do
-      str = "abc'def\"ghi\0jkl%mno"
+      str = "abc'def\"ghi\0jkl%mno".dup
       escaped = Mysql2::Client.escape(str)
       expect(escaped.encoding).to eql(str.encoding)