Fix 1.9 syntax error (with Error fixes)

[technoweenie]

This PR adds fixes some new failing tests added to #1086.

Changing the signature from:

# lib/faraday/error.rb
# Faraday::NilStatusError
-def initialize(_exc, response: nil)
+def initialize(_exc, response = nil)

Is a major change, but it's only used in one spot:

# lib/faraday/response/raise_error.rb
-raise Faraday::NilStatusError, response: response_values(env)
+raise Faraday::NilStatusError, response_values(env)

This PR updates it, and adds a deprecation warning.

>> Faraday::NilStatusError.new(response: {status: 1}).inspect
NOTE: Faraday::NilStatusError#unwrap_resp is deprecated; use  instead. It will be removed in or after version 1.0
Faraday::NilStatusError#unwrap_resp called from /Users/rick/p/faraday/lib/faraday/error.rb:121.
=> "#<Faraday::NilStatusError response={:status=>1}>"

>> Faraday::NilStatusError.new({status: 1}).inspect
=> "#<Faraday::NilStatusError response={:status=>1}>"

For the master branch, and eventually v1.0:

• Apply updated specs and fixes
• Ensure all error constructors are consistent
• Remove deprecation warnings
• Stop advertising ruby 1.9 support

[technoweenie]

The latest commit also fixes the %i issue reported in #1086 (comment).