Allow to register complex URI schemes for Adapters

Most notably, we want to use the plus character in URI schemes. By only
checking the first character of the scheme registration, we can better
fullfil our contract and better distinguish between schemes and class
names of registered Adapters.

lib/rackstash/adapter.rb

@@ -53,7 +53,7 @@ module Rackstash
           case matcher
           when String
             matcher = matcher.to_s
-            if matcher =~ /\A[a-z0-9]+\z/
+            if matcher =~ /\A[a-z0-9]/
               # If the matcher is a lower-case string, we assume it is a URL
               # scheme.
               adapter_schemes[matcher.downcase] = adapter_class

spec/rackstash/adapter_spec.rb

@@ -210,6 +210,7 @@ describe Rackstash::Adapter do
     context 'with a registered scheme' do
       before do
         described_class.register adapter, 'dummy'
+        described_class.register adapter, 'foo+dummy'
       end
 
       it 'creates an adapter from the scheme' do
@@ -218,6 +219,12 @@ describe Rackstash::Adapter do
         expect(described_class[raw_uri]).to be_an Rackstash::Adapter::BaseAdapter
       end
 
+      it 'can use a complex scheme' do
+        raw_uri = 'foo+dummy://example.com'
+        expect(adapter).to receive(:from_uri).with(URI(raw_uri)).and_call_original
+        expect(described_class[raw_uri]).to be_an Rackstash::Adapter::BaseAdapter
+      end
+
       it 'calls adapter.new if adapter.from_uri is not available' do
         plain_adapter = Class.new(Rackstash::Adapter::BaseAdapter)
         described_class.register plain_adapter, 'dummy'