Update mapping macro to support both case-sensitive and -insensitive

comparisons when matching column name to property name

Column to property name matching remains case-sensitive by default,
but the matching can be set to be case-insensitive as follows:

    # will match column names "foo" or "Foo" or "fOo" or FOO" etc.
    # to property name "foo"
    DB.mapping({ foo: String }, case_sensitive: false)

spec/mapping_spec.cr

@@ -63,6 +63,12 @@ class MappingWithConverter
   })
 end
 
+class MappingWithCaseInsensitivity
+  DB.mapping({
+    foo: {type: Int32, key: "C0"},
+  }, case_sensitive: false)
+end
+
 macro from_dummy(query, type)
   with_dummy do |db|
     rs = db.query({{ query }})
@@ -146,6 +152,10 @@ describe "DB.mapping" do
     expect_mapping("Zm9v,a", MappingWithConverter, {c0: "foo".to_slice, c1: "a"})
   end
 
+  it "should perform column name to property name match with case insensitivity" do
+    expect_mapping("1", MappingWithCaseInsensitivity, {foo: 1})
+  end
+
   it "should initialize multiple instances from a single resultset" do
     with_dummy do |db|
       db.query("1,a 2,b") do |rs|

src/db/mapping.cr

@@ -57,7 +57,7 @@ module DB
   # it and initializes this type's instance variables.
   #
   # This macro also declares instance variables of the types given in the mapping.
-  macro mapping(properties, strict = true)
+  macro mapping(properties, strict = true, case_sensitive = true)
     include ::DB::Mappable
 
     {% for key, value in properties %}
@@ -102,9 +102,9 @@ module DB
       {% end %}
 
       %rs.each_column do |col_name|
-        case col_name
+          case col_name{% if !case_sensitive %}.downcase{% end %}
           {% for key, value in properties %}
-            when {{value[:key] || key.id.stringify}}
+              when {{value[:key] || key.id.stringify}}{% if !case_sensitive %}.downcase{% end %}
               %found{key.id} = true
               %var{key.id} =
                 {% if value[:converter] %}