addModifiers(Iterable) for TypeSpec.Builder

[h908714124]

MethodSpec.Builder has two versions of the addModifiers method:

public Builder addModifiers(Modifier... modifiers) {...}
public Builder addModifiers(Iterable<Modifier> modifiers) {...}

but TypeSpec.Builder has only one of these.
This is counterintuitive.

[JakeWharton]

Yep, easy fix! Want to send a PR?

[h908714124]

@JakeWharton thanks, I'll leave it to someone who has signed the CLA

[BigJIU]

I have sent PR #873 , and just signed the CLA. Wish it could help.