Allow seek when underlying string is frozen (#121)

Fixes #119. Adds a test for this expectation.

Author: headius

ext/java/org/jruby/ext/stringio/StringIO.java

@@ -1356,8 +1356,6 @@ public IRubyObject seek(ThreadContext context, IRubyObject arg0, IRubyObject arg
     }
 
     private RubyFixnum seekCommon(ThreadContext context, int argc, IRubyObject arg0, IRubyObject arg1) {
-        checkModifiable();
-
         Ruby runtime = context.runtime;
 
         IRubyObject whence = context.nil;
@@ -1367,10 +1365,10 @@ private RubyFixnum seekCommon(ThreadContext context, int argc, IRubyObject arg0,
             whence = arg1;
         }
 
-        checkOpen();
-
         StringIOData ptr = this.getPtr();
 
+        checkOpen();
+
         boolean locked = lock(context, ptr);
         try {
             switch (whence.isNil() ? 0 : RubyNumeric.num2int(whence)) {

test/stringio/test_stringio.rb

@@ -483,6 +483,11 @@ def test_seek
     f.close unless f.closed?
   end
 
+  def test_seek_frozen_string
+    f = StringIO.new(-"1234")
+    assert_equal(0, f.seek(1))
+  end
+
   def test_each_byte
     f = StringIO.new("1234")
     a = []