Simplify expression in STPSectionHeaderView.swift

[JonathanDowning]

Summary

I've simplified an expression in this view to reduce the compile time from 13 seconds to 6 milliseconds.

Motivation

I analyze the compile time of my projects from time to time and discovered Stripe currently has a hotspot.

After making this innocuous change, the time to compile goes from:
12899.38ms stripe-ios/Stripe/STPSectionHeaderView.swift:124:10 instance method height(forButtonText:width:)
to:
6.05ms stripe-ios/Stripe/STPSectionHeaderView.swift:123:18 instance method height(forButtonText:width:)

Testing

The code is identical in functionality.

[davidme-stripe]

Thanks for catching this and cleaning it up! We'll publish it and credit you in the next update.

This code was automatically converted from Objective-C using a tool, and the results were a little odd in some places. We'll run through with -debug-time-function-bodies and try to fix a few of the other hotspots.

[JonathanDowning]

It's a pleasure @davidme-stripe! Thanks for all the hard work y'all do.