Handle plain absolute Windows paths gracefully for adapter creation

A absolute Windows path such as `C:/path.to/file.log` is a valid URL
with schema `C`. If a user specifies such a path, they likely want to
create a File adapter in this case. As such, we fallback from
`Adapter.adapter_by_uri_scheme` and create File adapter instance via
`Adapter.adapter_by_type`.

lib/rackstash/adapter.rb

@@ -112,6 +112,8 @@ module Rackstash
         return unless uri.scheme
 
         adapter_class = adapter_schemes.fetch(uri.scheme) {
+          # `uri` might look like a windows path, e.g. C:/path/to/file.log
+          return if ('a'..'z').include?(uri.scheme.downcase)
           raise ArgumentError, "No log adapter found for URI #{uri}"
         }
 

lib/rackstash/adapter/file.rb

@@ -118,7 +118,12 @@ module Rackstash
       # @param rotate (see #rotate=)
       # @param lock (see #lock=)
       def initialize(path, auto_reopen: true, rotate: nil, lock: false)
-        @base_path = ::File.expand_path(path).freeze
+        if path.is_a?(String) && path =~ %r{\A/?[a-z]:}i
+          path = path[1..-1] if path.start_with?('/')
+          @base_path = path.dup.freeze
+        else
+          @base_path = ::File.expand_path(path).freeze
+        end
 
         self.auto_reopen = auto_reopen
         self.rotate = rotate

spec/rackstash/adapter/file_spec.rb

@@ -35,8 +35,19 @@ RSpec.describe Rackstash::Adapter::File do
     it 'sets the base_path from the URI path' do
       expect(described_class.from_uri("file:#{logfile.path}").base_path)
         .to eql logfile.path
-      expect(described_class.from_uri("file://#{logfile.path}").base_path)
-        .to eql logfile.path
+
+      if Gem.win_platform?
+        expect(logfile.path).to match /\A[a-z]:/i
+
+        # For an absolute Windows path to be correctly identified in a URI, we
+        # need to add a third slash (since the drive letter doesn't have one on
+        # its own)
+        expect(described_class.from_uri("file:///#{logfile.path}").base_path)
+          .to eql logfile.path
+      else
+        expect(described_class.from_uri("file://#{logfile.path}").base_path)
+          .to eql logfile.path
+      end
     end
 
     it 'sets optional attributes' do