Function evaluation for segments with zero extent

Nobody has claimed this yet.

Hi!

I would like to understand the following behavior:

```Julia
julia> using QuadGK

julia> quadgk_count(x->x,0,0)
(0.0, 0.0, 15)
```

A 1D integral with the same upper and lower bound should always return zero. For `quadgk`, I assumed a shortcut that returns just zero without evaluating the integrand, but it is evaluated 15 times instead.

This is also the case for segmented integration:

```Julia
julia> using QuadGK

julia> quadgk_print(x->x,0,0,1)
f(0.0) = 0.0
f(0.0) = 0.0
f(0.0) = 0.0
f(0.0) = 0.0
f(0.0) = 0.0
f(0.0) = 0.0
f(0.0) = 0.0
f(0.0) = 0.0
f(0.0) = 0.0
f(0.0) = 0.0
f(0.0) = 0.0
f(0.0) = 0.0
f(0.0) = 0.0
f(0.0) = 0.0
f(0.0) = 0.0
f(0.5) = 0.5
f(0.39610752249605075) = 0.39610752249605075
f(0.6038924775039493) = 0.6038924775039493
f(0.025446043828620757) = 0.025446043828620757
f(0.9745539561713792) = 0.9745539561713792
f(0.004272314439593694) = 0.004272314439593694
f(0.9957276855604063) = 0.9957276855604063
f(0.12923440720030277) = 0.12923440720030277
f(0.8707655927996972) = 0.8707655927996972
f(0.06756778832011545) = 0.06756778832011545
f(0.9324322116798845) = 0.9324322116798845
f(0.2970774243113014) = 0.2970774243113014
f(0.7029225756886985) = 0.7029225756886985
f(0.20695638226615443) = 0.20695638226615443
f(0.7930436177338456) = 0.7930436177338456
(0.5, 0.0, 30)
```

The integrand is evaluated 15 times in the first segment before going to the segment with a finite extent.

In my opinion, a shortcut for segments with no extent could help to save some evaluations.

Start by reproducing the two examples in the issue and trace how quadgk handles each supplied segment, especially the zero-extent segment. Done means zero-extent segments return zero without evaluating the integrand, while the finite segment still produces the reported integral and error result.

Written by the indexing model from the issue text.