Change String#to_i to parse octals with prefix 0o

This is a breaking change.
Previously, numbers starting with 0 would be parsed as octals.
A previous change in Crystal requires octals start with 0o.
Now that is not the case, and they will be parsed as base-10.
The 0o prefix must be present to treat it as an octal.
This addresses parsing 0 with prefix: true raising an error.

Additionally, YAML parsing used the old style.
This updates the spec to handle the new parsing.

spec/std/string_spec.cr

@@ -235,7 +235,10 @@ describe "String" do
     it { "0x123abc".to_i(prefix: true).should eq(1194684) }
     it { "0b1101".to_i(prefix: true).should eq(13) }
     it { "0b001101".to_i(prefix: true).should eq(13) }
-    it { "0123".to_i(prefix: true).should eq(83) }
+    it { "0123".to_i(prefix: true).should eq(123) }
+    it { "0o123".to_i(prefix: true).should eq(83) }
+    it { "5".to_i(prefix: true).should eq(5) }
+    it { "0".to_i(prefix: true).should eq(0) }
     it { "123hello".to_i(strict: false).should eq(123) }
     it { "99 red balloons".to_i(strict: false).should eq(99) }
     it { "   99 red balloons".to_i(strict: false).should eq(99) }
@@ -247,6 +250,7 @@ describe "String" do
     it { expect_raises(ArgumentError) { "0b123".to_i } }
     it { expect_raises(ArgumentError) { "000b123".to_i(prefix: true) } }
     it { expect_raises(ArgumentError) { "000x123".to_i(prefix: true) } }
+    it { expect_raises(ArgumentError) { "000o89a".to_i(prefix: true) } }
     it { expect_raises(ArgumentError) { "123hello".to_i } }
     it { "z".to_i(36).should eq(35) }
     it { "Z".to_i(36).should eq(35) }

spec/std/yaml/schema/core_spec.cr

@@ -104,8 +104,9 @@ describe YAML::Schema::Core do
   it_parses_scalar "0b10110", 0b10110
 
   # integer (octal)
-  it_parses_scalar "00", 0
-  it_parses_scalar "0123", 0o123
+  it_parses_scalar "0o0", 0
+  it_parses_scalar "0o123", 0o123
+  it_parses_scalar "0755", 0o755
 
   # integer (hex)
   it_parses_scalar "0x0", 0
@@ -207,9 +208,10 @@ describe YAML::Schema::Core do
   it_raises_on_parse "!!float 'hello'", "Invalid float"
 
   # !!int
+  it_parses "!!int 0", 0
   it_parses "!!int 123", 123
   it_parses "!!int 0b10", 0b10
-  it_parses "!!int 0123", 0o123
+  it_parses "!!int 0o123", 0o123
   it_parses "!!int 0xabc", 0xabc
   it_parses "!!int -123", -123
   it_raises_on_parse "!!int 'hello'", "Invalid int"

src/string.cr

@@ -289,7 +289,7 @@ class String
   # Options:
   # * **whitespace**: if `true`, leading and trailing whitespaces are allowed
   # * **underscore**: if `true`, underscores in numbers are allowed
-  # * **prefix**: if `true`, the prefixes `"0x"`, `"0"` and `"0b"` override the base
+  # * **prefix**: if `true`, the prefixes `"0x"`, `"0o"` and `"0b"` override the base
   # * **strict**: if `true`, extraneous characters past the end of the number are disallowed
   #
   # ```
@@ -539,17 +539,23 @@ class String
       ptr += 1
 
       if prefix
-        case ptr.value.unsafe_chr
-        when 'b'
-          base = 2
-          ptr += 1
-        when 'x'
-          base = 16
-          ptr += 1
-        else
-          base = 8
-        end
-        found_digit = false
+        found_digit = case ptr.value.unsafe_chr
+                      when 'b'
+                        base = 2
+                        ptr += 1
+                        false
+                      when 'x'
+                        base = 16
+                        ptr += 1
+                        false
+                      when 'o'
+                        base = 8
+                        ptr += 1
+                        false
+                      else
+                        base = 10
+                        true
+                      end
       else
         found_digit = true
       end