fix: allow excluding accessor methods

[stephenplusplus]

While working on googleapis/nodejs-bigtable#794, I noticed some accessor methods were being unexpectedly called by this module, even though they were excluded. Example:

class Backup {
  // ...
  get expireDate() {
    throw new Error('Should not be called')
  }
}

// ...

promisifyAll(Backup, { exclude: ['expireDate'] });
// throws "Should not be called"

This is because of the ordering which this module checks what methods from a prototype should be excluded:

1. Is it a function?
2. Is it a stream, or the constructor? Something that can't be promsified?
3. Was it in the "exclude" array?

In this case, step 1 was what caused the error:

typeof Backup.prototype.expireDate
// throws "Should not be called"

The easiest way to solve this seemed to be to just bump step 3 up to step 1-- if the user wants it excluded, why do any other guessing?

[codecov]

Codecov Report

Merging #228 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #228   +/-   ##
=======================================
  Coverage   91.28%   91.28%           
=======================================
  Files           3        3           
  Lines         264      264           
  Branches       37       39    +2     
=======================================
  Hits          241      241           
  Misses         23       23           

Impacted Files	Coverage Δ
src/index.ts	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 90cc4c4...c8aa45c. Read the comment docs.