formula: make each instance have separate spec references

This fixes SoftwareSpec#owner being mixed up if there are multiple formula references.
This in turn fixes --force-bottle not working in some scenarios like `brew reinstall`.

Library/Homebrew/formula.rb

@@ -262,7 +262,7 @@ class Formula
   end
 
   def spec_eval(name)
-    spec = self.class.send(name)
+    spec = self.class.send(name).dup
     return unless spec.url
 
     spec.owner = self
@@ -1449,9 +1449,9 @@ class Formula
 
   # @private
   def ==(other)
-    instance_of?(other.class) &&
+    self.class == other.class &&
       name == other.name &&
-      active_spec == other.active_spec
+      active_spec_sym == other.active_spec_sym
   end
   alias eql? ==
 

Library/Homebrew/test/formula_spec.rb

@@ -567,6 +567,14 @@ describe Formula do
     expect(f.class.head).to be_kind_of(SoftwareSpec)
   end
 
+  specify "instance specs have different references" do
+    f = Testball.new
+    f2 = Testball.new
+
+    expect(f.stable.owner).to equal(f)
+    expect(f2.stable.owner).to equal(f2)
+  end
+
   specify "incomplete instance specs are not accessible" do
     f = formula do
       url "foo-1.0"