Minimize object creation on middleware call.

This patch isn't nearly as invasive as #773. Also it was tested against itself, not in a full Rails app. I'm contributing the perf scripts I wrote in another PR.

Notice that by memorizing methods, not only are we reducing objects created in omniauth, but also in hashie due to fewer method calls to various omniauth options.


## Bench

Before patch

```
Calculating -------------------------------------
                 ips       470 i/100ms
-------------------------------------------------
                 ips     4877.1 (±10.1%) i/s -      24440 in   5.061663s
2.1.2  ~/documents/projects/omniauth (master)
$ be rake perf:ips
Calculating -------------------------------------
                 ips       467 i/100ms
-------------------------------------------------
                 ips     4840.9 (±14.5%) i/s -      23817 in   5.068649s
2.1.2  ~/documents/projects/omniauth (master)
$ be rake perf:ips
Calculating -------------------------------------
                 ips       465 i/100ms
-------------------------------------------------
                 ips     4799.5 (±11.8%) i/s -      23715 in   5.019191s
```

After Patch

```
$ be rake perf:ips
Calculating -------------------------------------
                 ips       631 i/100ms
-------------------------------------------------
                 ips     6269.1 (±11.8%) i/s -      30919 in   5.014849s
2.1.2  ~/documents/projects/omniauth (master)
$ be rake perf:ips
Calculating -------------------------------------
                 ips       615 i/100ms
-------------------------------------------------
                 ips     6207.8 (±10.3%) i/s -      31365 in   5.106172s
2.1.2  ~/documents/projects/omniauth (master)
$ be rake perf:ips
Calculating -------------------------------------
                 ips       522 i/100ms
-------------------------------------------------
                 ips     6155.8 (±9.3%) i/s -      30798 in   5.048455s
```



## Memory Bench

100 requests without patch


```
allocated objects by gem
-----------------------------------
hashie-3.3.1 x 6901
rack-1.5.2 x 5300
omniauth/lib x 3637 <====================
ruby-2.1.2/lib x 301
other x 300
```

100 requests with patch 

```
allocated objects by gem
-----------------------------------
rack-1.5.2 x 4800
hashie-3.3.1 x 4701
omniauth/lib x 1037 <====================
ruby-2.1.2/lib x 301
other x 300
```

Results in `(3637  - 1037 )/ 3637.0 * 100  # => 71 %` less objects.