Respect custom receivers

[shaydewael]

Summary

Addresses #235

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ff81e06). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #271   +/-   ##
=========================================
  Coverage          ?   59.95%           
=========================================
  Files             ?        7           
  Lines             ?      492           
  Branches          ?      136           
=========================================
  Hits              ?      295           
  Misses            ?      169           
  Partials          ?       28

Impacted Files	Coverage Δ
src/App.ts	70.88% <100%> (ø)

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[seratch]

We can change these lines to make it more easy-to-understand:

// Check for required arguments of ExpressReceiver
if (signingSecret !== undefined && receiver === undefined) {
    this.receiver = new ExpressReceiver({ signingSecret, logger, endpoints });
} else if (receiver === undefined) {
    // Check for custom receiver

How about this?

// Check for required arguments of ExpressReceiver
if (receiver !== undefined) {
    // NOTE: signingSecret would be ignored in this case
    this.receiver = receiver;
} else {
    // no custom receiver given here
    if (signingSecret === undefined) {
        throw errorWithCode(
            'Signing secret not found, so could not initialize the default receiver. ' +
            'Set a signing secret or use a custom receiver.',
            ErrorCode.AppInitializationError,
        );
    } else {
        this.receiver = new ExpressReceiver({ signingSecret, logger, endpoints });
    }
}