Fix deduplicate_knots!()

[chunjiw]

Replace 1 with firstindex(knots) to make the code more robust.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 :tada:

Comparison is base (2c383dd) 87.82% compared to head (6bd250f) 87.85%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #559 +/- ## ========================================== + Coverage 87.82% 87.85% +0.03% ========================================== Files 28 28 Lines 1865 1878 +13 ========================================== + Hits 1638 1650 +12 - Misses 227 228 +1 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaMath/Interpolations.jl/pull/559?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath) | Coverage Δ | | |---|---|---| | [src/gridded/gridded.jl](https://app.codecov.io/gh/JuliaMath/Interpolations.jl/pull/559?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath#diff-c3JjL2dyaWRkZWQvZ3JpZGRlZC5qbA==) | `98.41% <100.00%> (ø)` | | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/JuliaMath/Interpolations.jl/pull/559/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath)

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[mkitti]

Thanks