fixes #7173 fallback to safe-buffer in the browser

[ghost]

re: #7173 global.Buffer is undefined in the browser. This change sets NodeBuffer to safe-buffer in this environment.

all current tests pass:

mongoose>: npm test -- --no-warnings

> [email protected] test /Users/lineus/dev/node/src/mongoose
> mocha --exit test/*.test.js test/**/*.test.js "--no-warnings"


 You're not testing shards!
 Please set the MONGOOSE_SHARD_TEST_URI env variable.
 e.g: `mongodb://localhost:27017/database
 Sharding must already be enabled on your database


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  ․․․․․․․․․,․․,․․,․,,․․,․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․
  ,,․․․․․․․․․․․․․․,,,,․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․
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  ․․․․․․․․․․․․․․,,․․․․․․․․․․․․․․․․․․․․․․․,․․․․․․․․,․,․․․․․․․․․․․․․․․․․․․․․․․․․․․
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  ,,,,,,,․․․․․․․․․․․․․․․․․․․․․․

  2098 passing (51s)
  37 pending

mongoose>:

[vkarpov15]

Good catch @lineus .