fix(buildURL): ensure operation is idempotent

[ericdeansanchez]

The purpose of this PR is to ensure the buildSrcSet operation is
idempotent. Prior to this PR, code modified the input-parameters,
params, causing identical calls to buildSrcSet to produce different
results.

Now, the input params are copied into a new queryParams object and
this object is passed to callers requiring params. A test has been
written to show that the behavior has changed from the behavior detailed
in issue #158––the input params object remains unchanged after calling
buildSrcSet and that calling buildSrcSet multiple times produces
the same result (given the same inputs).

I also ripgrep'd through the repo with rg 'params.[:alpha:]' and with
rg 'params\[' to ensure params is never mutated (i.e. it never
appears on the left hand side of any expression).

Tests have been added to ensure that _buildParams:

• does not modify its input-argument

Tests have been added to ensure that calling buildURL:

• is an idempotent operation with empty args
• is an idempotent operation with args
• does not modify empty args
• does not modify its args (non-empty)

Tests have been added to ensure that buildSrcSet:

• does not modify input params and is idempotent

Closes #158

[frederickfogerty]

Looks good overall! Good work @ericdeansanchez!

[ericdeansanchez]

🎉 This PR is included in version 0.3.0-beta.1 🎉

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

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