Autocorrect for Performance/Sum drops initial value if it is not a number literal

[mvz]

When autocorrecting, the new Performance/Sum cop should take the initial value provided to inject or reduce and pass that as the first argument to sum, even when it is not a number literal.

This is corrected correctly:

foo.inject(23, :+)

But this is not:

foo.inject(bar, :+)

Expected behavior

The code

foo.inject(bar, :+)

should be corrected to

foo.sum(bar)

Also, more complex code like the following real-world example:

paid_bills.reduce(AmountWithVAT.new, :+)

should be corrected to

paid_bills.sum(AmountWithVAT.new)

Actual behavior

The two examples given above are corrected to

foo.sum

and

paid_bills.sum

Steps to reproduce the problem

Let rubocop-performance autocorrect the examples given above

RuboCop version

0.90.0 (using Parser 2.7.1.4, rubocop-ast 0.3.0, running on ruby 2.7.1 x86_64-linux)

rubocop-performance is at version 1.8.0.