Convert source to Buffer when getting asset's hash

[3846masa]

R: @jeffposnick @philipwalton

Fixes #1916

Description of what's changed/fixed.

asset.source() will return string or ArrayBuffer (Ref.) (or Buffer via some loaders), but Hash.update() shouldnot receive ArrayBuffer (Ref.).

Because of that, this PR enables to convert source to Buffer when getting asset's hash.

[coveralls]

Coverage remained the same at 82.7% when pulling 2a902da on 3846masa-tmp:fix-get-asset-hash into b56962a on GoogleChrome:master.

[jeffposnick]

Thank you very much for your contribution!

I would like to add in a unit test to catch this scenario in the future, since we obviously did not have a test for this already. If you don't mind, I'll push the commit to your branch and include it in this PR.

To confirm, you were using https://github.com/ballercat/wasm-loader when you ran into this issue, correct?

[jeffposnick]

...or is it just the native webpack v4 WASM support that triggers this, like https://github.com/webpack/webpack/tree/master/examples/wasm-simple

[jeffposnick]

Nevermind! I found your reproduction link to from the original issue, and I'll use that as the basis of the test: https://gist.github.com/3846masa/41a96d1fd4c192d973d2c5f772144d1f

[jeffposnick]

I confirmed via the new test case that the test previously failed, and with the updated code, it now passes. Thanks again for the contribution!