fix edge case in truncate function allowing too long slugs

[Redlinkk]

Fixes #72

I'm not using operations on strings to avoid unnecessary allocations.
I've added a test case for the fixed issue.

[codecov]

Codecov Report

Merging #74 (c05c02d) into master (452645f) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #74   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          150       142    -8     
=========================================
- Hits           150       142    -8     

Impacted Files	Coverage Δ
slug.go	100.00% <100.00%> (ø)

[matrixik]

Hello, sorry it took me so long to review it (burn out issues).

I improved tests to have more edge cases like this and rebased with master branch.
Looks like when fixing this problem you introduced another one: now it's cutting word that should not be cut.

Thank you for catching this. I will wait with releasing new version until this is merged.

[matrixik]

--- FAIL: TestSlugMakeSmartTruncate (0.00s)
    slug_test.go:327: 4. MaxLength = 16; Make("Dobroslaw Zybort") = "dobroslaw"; want "dobroslaw-zybort"
    slug_test.go:327: 13. MaxLength = 6; Make("abc-de") = "abc"; want "abc-de"
    slug_test.go:327: 15. MaxLength = 6; Make("abc-de-fg") = "abc"; want "abc-de"
    slug_test.go:327: 18. MaxLength = 9; Make("abc-de-fg") = "abc-de"; want "abc-de-fg"
FAIL