Don't require the string be modifiable to seek

seek only modifies the StringIO pos, and should work when the
underlying String is frozen.

Fixes #119

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)) {