add dbt_utils/sqlserver__generate_surrogate_key macro to fix concat bug

#97

macros/dbt_utils/sql/generate_surrogate_key.sql

@@ -0,0 +1,29 @@
+{%- macro sqlserver__generate_surrogate_key(field_list) -%}
+
+{%- if var('surrogate_key_treat_nulls_as_empty_strings', False) -%}
+    {%- set default_null_value = "" -%}
+{%- else -%}
+    {%- set default_null_value = '_dbt_utils_surrogate_key_null_' -%}
+{%- endif -%}
+
+{%- set fields = [] -%}
+
+{%- for field in field_list -%}
+
+    {%- do fields.append(
+        "coalesce(cast(" ~ field ~ " as " ~ dbt.type_string() ~ "), '" ~ default_null_value  ~"')"
+    ) -%}
+
+    {%- if not loop.last %}
+        {%- do fields.append("'-'") -%}
+    {%- endif -%}
+
+{%- endfor -%}
+
+{%- if fields|length > 1 %}
+    {{ dbt.hash(dbt.concat(fields)) }}
+{%- else -%}
+    {{ dbt.hash(fields[0]) }}
+{%- endif -%}
+
+{%- endmacro -%}