number_to_currency now returns a String instead of writing to the view

[jwoertink]

Purpose

fixes #790

Description

In a recent PR, most of the page helpers were converted to return a String instead of writing to the view via text. The rule is now if the method generates HTML, it writes to the view, otherwise it should return a String. This method was missed during that commit.

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test

Specs pass when settings avram back to 0.9.4. Since Avram renamed Field and Form stuff recently, and Lucky pulls from Avram master, specs fail.

[edwardloveall]

This is all fine, lucky just needs to be updated: #810