Character highlight broken by case-mismatches

[dela3499]

The potential completions are case-insensitive, but a case mismatch between the input and the potential matches breaks things.

Example:
input: "he"
potential matches:
"hello" <- "he" is highlighted
"Heck!" <- nothing is highlighted, the initial case mismatch between the "h" and "H" breaks character highlighting.

[blq]

Same issue reported here: #63
A PR is pending #44