Resize capacity for fstring

When a string is #frozen, it's capacity is resized to fit (if it is much
larger), since we know it will no longer be mutated.

    > puts ObjectSpace.dump(String.new("a"*30, capacity: 1000))
    {"type":"STRING", "class":"0x7feaf00b7bf0", "bytesize":30, "capacity":1000, "value":"...
    > puts ObjectSpace.dump(String.new("a"*30, capacity: 1000).freeze)
    {"type":"STRING", "class":"0x7feaf00b7bf0", "frozen":true, "bytesize":30, "value":"...

(ObjectSpace.dump doesn't show capacity if capacity is equal to bytesize)

Previously, if we dedup into an fstring, using String#-@, capacity would
not be reduced.

    > puts ObjectSpace.dump(-String.new("a"*30, capacity: 1000))
    {"type":"STRING", "class":"0x7feaf00b7bf0", "frozen":true, "fstring":true, "bytesize":30, "capacity":1000, "value":"...

This commit makes rb_fstring call rb_str_resize, the same as
rb_str_freeze does.

Closes: https://github.com/ruby/ruby/pull/2256

string.c

@@ -321,6 +321,9 @@ rb_fstring(VALUE str)
 	return str;
     }
 
+    if (!OBJ_FROZEN(str))
+        rb_str_resize(str, RSTRING_LEN(str));
+
     fstr = register_fstring(str);
 
     if (!bare) {

test/-ext-/string/test_capacity.rb

@@ -37,4 +37,23 @@ class Test_StringCapacity < Test::Unit::TestCase
     open(__FILE__) {|f|s = f.read(1024*1024)}
     assert_operator(capa(s), :<=, s.bytesize+4096)
   end
+
+  def test_literal_capacity
+    s = "I am testing string literal capacity"
+    assert_equal(s.length, capa(s))
+  end
+
+  def test_capacity_frozen
+    s = String.new("I am testing", capacity: 1000)
+    s << "fstring capacity"
+    s.freeze
+    assert_equal(s.length, capa(s))
+  end
+
+  def test_capacity_fstring
+    s = String.new("I am testing", capacity: 1000)
+    s << "fstring capacity"
+    s = -s
+    assert_equal(s.length, capa(s))
+  end
 end