Unit Tests for Common Functions: Lib

[kibanamachine]

Original comment by @cqliu1:

We need tests for the following lib functions:
- [ ] arg
- [ ] ast

• dataurl
• errors
• expression_form_handlers
  - [ ] fetch_image
• fetch
  - [ ] find_in_object
  - [ ] fn
  - [ ] functions_registry
• get_field_type
• get_legend_config
  - [ ] get_type
  - [ ] grammar
• handlebars
• hex_to_rgb
• httpurl
  - [ ] latest_change
• palettes
• pivot_object_array
  - [ ] registry
• resolve_dataurl
  - [ ] type
  - [ ] types_registry
• unquote_string
• url