BUG: NumericIndex.insert does not preserve dtype

[jbrockmendel]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the master branch of pandas.

Reproducible Example

import pandas as pd

idx = pd.NumericIndex([1.0, 2.0, 3.0] dtype='int32')
result = idx.insert(1, 4.0)

assert result.dtype == idx.dtype  # <- nope!

### Issue Description

cc @topper-123 

### Expected Behavior

Should preserve type and dtype

### Installed Versions

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Replace this line with the output of pd.show_versions()

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