Fix BlockHeader.prototype.validate() bug

[vpulim]

Fixes a this instead of self bug in BlockHeader.prototype.validate().

Needs tests, but I wanted to submit a PR to highlight the fix.

[coveralls]

Coverage remained the same at 73.234% when pulling 22729b8 on validate-fix into 16aba3e on master.

[axic]

This looks good to me. The code is within a callback, where the outer scope's this wouldn't be valid.

[vpulim]

Thanks @axic. Once this is merged, can we do a 2.1.0 release so that ethereumjs/ethereumjs-blockchain#59 can be merged?

[holgerd77]

@vpulim Hi Vinay, just had a short look, does this really justify a 2.1.0 release or is 2.0.1 enough (I have not strong opinion)?

[axic]

Definitely 2.0.1, 2.1.0 would mean a breaking change in semver and thus users wouldn't get the bugfix.

[vpulim]

@axic, @holgerd77 Yes, 2.0.1 makes more sense since it’s just a bug fix. Thanks.

[holgerd77]

Ok, just released this as v2.0.1: https://github.com/ethereumjs/ethereumjs-block/releases/tag/v2.0.1