Make weighted gauss(...) function use the same floating point type as endpoints

[frankier]

I assume this if a bugfix to the intended behavior. The problem is that 2.0 is a Float64 and will therefore promote the whole expression to Float64 from Float32.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b14ea2f) 97.62% compared to head (7ed233c) 97.62%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #79 +/- ## ======================================= Coverage 97.62% 97.62% ======================================= Files 5 5 Lines 337 337 ======================================= Hits 329 329 Misses 8 8 ``` | [Impacted Files](https://codecov.io/gh/JuliaMath/QuadGK.jl/pull/79?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath) | Coverage Δ | | |---|---|---| | [src/weightedgauss.jl](https://codecov.io/gh/JuliaMath/QuadGK.jl/pull/79?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath#diff-c3JjL3dlaWdodGVkZ2F1c3Muamw=) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath)

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