Use String#encode instead of Iconv

Modify to use String#encode instead of Iconv when ASCII Encodings.
There are two reasons:

1. @iconv was defined only encoding_to is NON_ASCII_ENCODINGS
2. Iconv is replaced from ruby-2.0

But when NON_ASCII_ENCODINGS, String#encoding cannot convert then
use Iconv as in the past.
updated routes to rm deprecation warning
Merge pull request #2077 from afxjzs/master

updated routes to rm deprecation warning when running rake test
Support Japanese encodings.
Merge pull request #2078 from mtfuji/master

Support Japanese encodings.
Friendlier error if filter is not found

If user sets, e.g., `filters [:name, :manager]`,
and manager is not set to `filterable true`,
RailsAdmin currently raises a NoMethodError
when trying to call `.type` on nil.

This change raises a more descriptive error
to help the user find the problem
Merge pull request #2079 from nathanl/friendlier_filter_error

Friendlier error if filter is not found
Move navigation out into partial so it can be easily overridden
Merge pull request #2081 from thefrontiergroup/feature/easily_replace_header

Move navigation out into partial so it can be easily overridden
Bump version to 0.6.4
Disallow font-awesome-rails versions >= 5
Disallow sass-rails versions >= 5
Don't package test files with gem
Bump version to 0.6.5
Disallow sass-rails versions >= 6
use &times; instead of hardcoded unicode times
Merge pull request #2084 from n3bulous/bug/replace-utf8-x-with-times-entity

use &times; instead of hardcoded unicode times
Fixed number translation in filter
Merge pull request #2093 from Neschur/fix_number_translation_in_filterbox

Fixed number translation in filter
Merge remote-tracking branch 'upstream/pr/2075'