Fix test_ocptr_ary_assign fails on ruby-1.8

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2570 f4f36d40-9719-0410-9868-dde021a23eda
Fix missing symbol rb_enc_str_new_cstr on ruby-2.0

rb_enc_str_new_cstr() in include/ruby/encoding.h is provided
by ruby-2.1 or later. use rb_enc_str_new().

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2571 f4f36d40-9719-0410-9868-dde021a23eda
Add argument vaidation to OSX.load_plist()

OSX.load_plist() accepts String, OSX::NSString and OSX::NSData.

  OSX.load_plist(nil) # => ArgumentError
  OSX.load_plist(broken_input) # => RuntimeError

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2572 f4f36d40-9719-0410-9868-dde021a23eda
Allow running tests with `minitest`, standard library of ruby-2.0

test-unit            minitest
---------------------------------------------
assert_true(cond)    assert_equal(true, cond)
assert_false(cond)   assert_equal(false, cond)

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2573 f4f36d40-9719-0410-9868-dde021a23eda