Don't log when overwriting Mash keys

When we switched to using `#respond_to?` to detecting whether to log
a Mash collision, we started reporting when we were overwriting keys
that already exist in the Mash. This is a poor experience because it
causes extra warnings (as in #414) or, in the worst case, causes an
"undefined method" error (as in #413).

This change fixes that problem and benchmarks to ensure we're not
appreciably regressing performance. The results of two benchmarks are
below:

```
bundle exec ruby benchmark/mash_method_access.rb:

Warming up --------------------------------------
  before    92.456k i/100ms
Calculating -------------------------------------
  before      1.290M (± 4.4%) i/s -      6.472M in   5.028183s

Pausing here -- run Ruby again to measure the next benchmark...

Warming up --------------------------------------
    after    92.941k i/100ms
Calculating -------------------------------------
    after      1.326M (± 5.4%) i/s -      6.692M in   5.060756s

Comparison:
   after:  1326239.2 i/s
  before:  1289624.0 i/s - same-ish: difference falls within error
```

and

```
within spec/integrations/omniauth,
bundle exec rake perf:ips

Warming up --------------------------------------
  before     1.260k i/100ms
Calculating -------------------------------------
  before     13.114k (± 4.2%) i/s -     66.780k in   5.101689s

Pausing here -- run Ruby again to measure the next benchmark...

Warming up --------------------------------------
    after     1.299k i/100ms
Calculating -------------------------------------
    after     13.149k (± 4.0%) i/s -     66.249k in   5.046630s

Comparison:
   after:    13148.9 i/s
  before:    13113.8 i/s - same-ish: difference falls within error
```

Closes #413
Closes #414

1 files changed, 6 insertions, 1 deletions

diff --git a/lib/hashie/mash.rb b/lib/hashie/mash.rb
index cc8cb20..390a731 100644
--- a/lib/hashie/mash.rb
+++ b/lib/hashie/mash.rb
@@ -145,7 +145,7 @@ module Hashie
     def custom_writer(key, value, convert = true) #:nodoc:
       key_as_symbol = (key = convert_key(key)).to_sym
 
-      log_built_in_message(key_as_symbol) if respond_to?(key_as_symbol)
+      log_built_in_message(key_as_symbol) if log_collision?(key_as_symbol)
       regular_writer(key, convert ? convert_value(value) : value)
     end
 
@@ -348,5 +348,10 @@ module Hashie
         'property. You can still access the key via the #[] method.'
       )
     end
+
+    def log_collision?(method_key)
+      respond_to?(method_key) && !self.class.disable_warnings? &&
+        !(regular_key?(method_key) || regular_key?(method_key.to_s))
+    end
   end
 end