Use utf-8 as default charset

[kommen]

This fixes serving utf8 encoded files, e.g. plotlyjs, which contains:

var ε = 1e-6, ε2 = ε ε, π = Math.PI, τ = 2 π, τε = τ - ε, halfπ = π / 2, d3_radians = π / 180, d3_degrees = 180 / π;

[pandeiro]

Thanks!