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, 8 insertions, 0 deletions

diff --git a/spec/hashie/mash_spec.rb b/spec/hashie/mash_spec.rb
index 6b9882b..162d982 100644
--- a/spec/hashie/mash_spec.rb
+++ b/spec/hashie/mash_spec.rb
@@ -142,6 +142,14 @@ describe Hashie::Mash do
       expect(logger_output).to match('Hashie::Mash#trust')
     end
 
+    it 'can set keys more than once and does not warn when doing so' do
+      mash = Hashie::Mash.new
+      mash[:test_key] = 'Test value'
+
+      expect { mash[:test_key] = 'A new value' }.not_to raise_error
+      expect(logger_output).to be_blank
+    end
+
     it 'does not write to the logger when warnings are disabled' do
       mash_class = Class.new(Hashie::Mash) do
         disable_warnings